App-V 5 Server, F5 Load Balancers, and Kerberos

More fun today with Kerberos and load balancers.  Today’s challenge related to getting the Microsoft App-V publishing server to work with an F5 load balancer in a Layer 4/n-Path/DSR configuration.  Everything was working when I was accessing the individual server nodes, but when I switched to using the load balanced name and address, authentication started to fail.

After lots of log searching I eventually tried a wire trace, and found the following Kerberos error in the response from the App-V server to the App-V client:
KRB5KRB_AP_ERR_MODIFIED

Lots of different resources helped here:

  • This TechNet page explains various Kerberos errors and why they might occur:
    http://blogs.technet.com/b/askds/archive/2008/06/11/kerberos-authentication-problems-service-principal-name-spn-issues-part-3.aspx

    Of note is the scenario where the account handling the authentication request does not hold the SPN for which the request was made.  I set the SPN for my IIS application pool identity, but further analysis of the error packet shows that it was handled by my App-V server machine account, not the service account.  Augh!  Why?

  • This thread on TechNet Social was the biggest help:

    http://social.technet.microsoft.com/Forums/en-US/2b39e2b8-aba1-4e96-b18f-c5bcb9f12687/load-balancing-two-appv-50-servers-the-publishing-service-is-not-able-to-contact-the-management

    The user posted all of the steps they followed in configuring IIS and the service account SPN, including the tidbit:
    changed the authentication of the “Management Service” web site to useAppPoolCredentials=”true”
    I have never used this particular setting, so I dug into it…

  • The following MSDN article explains the IIS 7.0 feature of “kernel authentication”, how it affects the need for SPN entries, and its interplay with application pool identity accounts:
    http://blogs.msdn.com/b/webtopics/archive/2009/01/19/service-principal-name-spn-checklist-for-kerberos-authentication-with-iis-7-0.aspx

    Basically, with kernel-mode authentication, the SYSTEM account will handle all Kerberos authentication by default.  This explains why we were seeing Kerberos errors in the communications with the App-V client… the IIS pool identity account was not handling Kerberos delegation!

    Of special interest is this statement:
    Either:
    Disable Kernel mode authentication and follow the general steps for Kerberos as in the previous IIS 6.0 version.
    Or,
    [Recommended for Performance reasons]
    Let Kernel mode authentication be enabled and the Application pool’s identity be used for Kerberos ticket decryption. The only thing you need to do here is:
    1. Run the Application pool under a common custom domain account.
    2. Add this attribute “useAppPoolCredentials” in the ApplicationHost.config file.

  • This TechNet page documents how to configure Kerberos auth in IIS, and mentions the use of the IIS appcmd.exe to set the “useAppPoolCredentials” option:
    http://technet.microsoft.com/en-us/library/dd759186.aspx
    Included is the exact command line required to set the value to true:
     appcmd.exe set config -section:system.webServer/security/authentication/windowsAuthentication -useAppPoolCredentials:true
    (But the page does not really tell you what it is for, which is where the MSDN article comes in handy.)

So, Kerberos under IIS 7 and later has some nuances not present in IIS 6.  I wonder how I did not encounter this before?

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